docs: def log(self, session):
      """Logs training progress."""
      logging.info('Train [%s/%d], step %d (%.3f sec) %.1f '
                   'global steps/s, %.1f local steps/s', self.task.type,
                   self.task.index, self.global_step,
                   (self.now - self.start_time),
                   (self.global_step - self.last_global_step) /
                   (self.now - self.last_global_time),
                   (self.local_step - self.last_local_step) /
                   (self.now - self.last_local_time))
      self.last_log = self.now
      self.last_global_step, self.last_global_time = self.global_step, self.now
      self.last_local_step, self.last_local_time = self.local_step, self.now